linux-kernel - Re: [PATCH v3] sched/fair: unlink misfit task from cpu overutilized

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Message-ID: <69457060-ede6-c805-af1d-a6e2b05fd55e@arm.com>
Date:   Wed, 18 Jan 2023 16:17:31 +0000
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...nel.org,
        peterz@...radead.org, rafael@...nel.org, viresh.kumar@...aro.org,
        vschneid@...hat.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, lukasz.luba@....com, wvw@...gle.com,
        xuewen.yan94@...il.com, han.lin@...iatek.com,
        Jonathan.JMChen@...iatek.com
Subject: Re: [PATCH v3] sched/fair: unlink misfit task from cpu overutilized

On 17/01/2023 16:38, Qais Yousef wrote:
> On 01/16/23 09:07, Dietmar Eggemann wrote:
> 
> [...]
> 
>> Not sure if people get what `performance requirements` mean here? I know
>> we want to use `performance` rather `bandwidth hint` to describe what
>> uclamp is. So shouldn't we use `utilization but also uclamp`?
> 
> We do have the uclamp doc now which explains this, no? I'm not keen on
> utilization because it's an overloaded term. In the context of uclamp

And `performance` isn't ? ;-) True, the doc refers to uclamp as a
`performance requirements`.

> - utilization _signal_ in the scheduler is used to indicate performance
> requirements of a workload, no?

I was referring to:

 4569 static inline int task_fits_cpu(struct task_struct *p, int cpu)
 4570 {
 4571   unsigned long uclamp_min = uclamp_eff_value(p, UCLAMP_MIN);
 4572   unsigned long uclamp_max = uclamp_eff_value(p, UCLAMP_MAX);
 4573   unsigned long util = task_util_est(p);
 4574   /*
 4575   * Return true only if the cpu fully fits the task requirements,
 4576   * which include the utilization but also the performance.
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 4577   */
 4578   return (util_fits_cpu(util, uclamp_min, uclamp_max, cpu) > 0);
 4579 }

So here we explicitly talk about `utilization` (util_avg/util_est)
versus `uclamp (max/min)` and the latter is referred as `performance`.
You're right, we shouldn't refer to `uclamp (min/max)` as `utilization`
either.

In other places we use:

select_idle_capacity()

/* This CPU fits with all capacity and performance requirements */
                          ^^^^^^^^     ^^^^^^^^^^^^^^^^^^^^^^^^

`capacity` is probably equal `utilization`? and `performance
requirements` stand for `uclamp (min/max)`.

/* Only the min performance (i.e. uclamp_min) doesn't fit */
            ^^^^^^^^^^^^^^^
here we link `min performance` explicitly to `uclamp_min`.

/* Look for the CPU with highest performance capacity.
                                 ^^^^^^^^^^^^^^^^^^^^
I guess this stands for `cap_orig - thermal_load_avg()`

feec()

/* Both don't fit performance (i.e. uclamp_min) but best energy cpu has
                  ^^^^^^^^^^^  ^^^^^^^^^^^^^^^
better performance. */
^^^^^^ ^^^^^^^^^^^

Here I assume `better performance` stands for higher `cap_orig -
thermal_pressure', not for `uclamp min or max`?

---

IMHO, referring to `uclamp (min/max)` as `performance (min/max)
hint/(requirement)` is fine as long as it's done consistently in
comments and the alias is not used for other items.

> 
> Using 'uclamp hint' if you found it really confusing, is fine by me. But I'd
> rather steer away from 'bandwidth' or 'utilization' when describing uclamp and
> its intention.
> 
> I like using performance requirements because it enforces what this hint
> actually means.